Having a starter Pokémon is not very important at all. It is recommended to put it in your box when starting play.

Steps

1

Train the other Pokémon to level 50 or higher. It is simple: Make sure you have Pokémon that have moves that are super effective against the opponent's Pokémon.

2

Try the following line-up.'

Gloom (lvl 52)

Walrein (lvl 54)

Ninetails (lvl 51)

Kadabra (lvl 56)

Machoke (lvl 55)

Raichu (lvl 58)

3

Beat the elite four. If you have a line-up as described in the previous step, it should be simple to beat the elite four. Note that you can even have them in the high 40's, (48-52), and still manage to beat the elite four.

4

Remember to save after each opponent beaten. This is very important or you might have to start all over again.